The catalytic conversion of the noxious components in vehicular exhaust gases is in current use for overcoming air pollution. The catalysts for the various forms of converters are, at this time, being manufactured and supplied in two general forms, namely: (1) as catalyst coated rigid skeletal monoliths, honeycomb type of elements, where there are a multiplicity of longitudinal passageways in each unit in order to provide a high surface area; and (2) as confined masses or beds of catalyst coated pellets which may be generally of spherical or cylindrical form with diameters and lengths of about 1/8 inch.
The present invention relates to ceramic monolithic catalyst support and more particularly, to an improved method for treating the catalyst support in a controlled manner to effect a high surface area coating thereon which is reproducible between supports, using apparatus adapted to mass production requirements. It is essential from the standpoint of costs and attainment of the desired level of catalyst performance in a production process of the type herein described to be able to closely control the coating operation for repeatable results between succeeding supports, in order to minimize the amount of coating applied, especially where the catalytically active material, e.g., platinum, palladium, rhodium, is codeposited with the high surface area material.
The rigid monolithic, honeycomb structures are typically made from ceramics which comprise refractory crystalline materials such as sillimanite, magnesium silicates, zircon, petalite, spodumene, cordierite, aluminosilicates, mullite, or combinations thereof, etc., cordierite being preferred because of the generally lower coefficient of thermal expansion achievable, as shown in U.S. Pat. No. 3,954,672 to Somers et al. While such materials are generally considered to have a porous surface, it is generally advisable to provide a coating over the honeycomb structure, e.g., active alumina, to achieve a high surface area for distribution of the catalytically active component. These monolithic, substantially catalytically-inactive crystalline support members have been described in prior art patents, as for example in Keith et al U.S. Pat. No. 3,565,830, and the British Pat. No. 931,096 and its Japanese Pat. No. 292,558, such that it is not deemed necessary to describe them in detail herein.
Typically, and by way of example only, the catalytic component will comprise one or more of the noble and base metals and metal oxides of Groups IB, VB, VIB, VIIB and VIII of the Periodic Table, particularly copper, vanadium, chromium, manganese, iron, cobalt, nickel, platinum, palladium, rhodium and ruthenium, with a component being used singly or in combination with one or more of other active components.
While methods and apparatus are known in the art for coating a monolith support with a highly porous, fine grained, high surface area, refractory coating e.g., gamma alumina, the methods have either been such as to not lend themselves to mass production requirements or the apparatus has been costly and the process complex. The identified U.S. patent to Keith discloses the immersion of the monolith in the slurry with agitation to coat the passageways fully, followed by shaking and gentle blowing with air to remove excess coating slurry and open the plugged passageways. Such technique is both time consuming and uncertain in its results and blow-back of slurry is not uncommon. The U.S. Patent to Hoyer et al U.S. Pat. No. 4,039,482 discloses a method for coating whereby the monolith is encapsulated in a sealed chamber in which it is successively subjected to vacuum to de-gas the pores of the structure, followed by contact with a flowing stream of the coating slurry which flow is cut-off and, with the support submerged completely in the slurry in the chamber, pressure is applied to the slurry to force it into the support pores, this being followed by gravity draining and the application of air under pressure in the chamber to blow-down the excess slurry and unplug the passageways. A still further method for coating is disclosed in U.S. Pat. No. 3,873,350 to Dwyer et al comprising the steps of heating the support, immersing the support in the coating slurry, removing the coated support and draining excess slurry while shaking the support, and rotating the support about a substantially horizontal axis while blowing air through the passageways. U.S. Pat. No. 2,867,544 to Hall, Jr. discloses a method for coating the inner wall surface of or filling small diameter glass tubes wherein the tubes are first heated for obtaining adherence to and a continuous coating of the wall surface by the coating material, each tube being mounted between a pair of axially aligned holding members, the bottom members being connected to the coating material source, the top member being connected to a vacuum source which acts to draw the coating material up into the tube, the flow being interrupted by opening the bottom member to air.